Exhaust-gas recirculation (EGR) helps to reduce NOx emissions from an engine by controlling peak combustion temperatures. EGR works by diluting the intake air charge with exhaust gas, thereby reducing its oxygen content. When diluted air is used in place of ordinary air to support combustion in the engine, lower peak combustion temperatures result.
EGR can also improve fuel economy in gasoline engines. At medium and high loads, fuel economy is improved due to knock mitigation, allowing for more efficient combustion phasing, reduced heat loss to the engine coolant, and lower exhaust temperatures, which in turn reduce the need for enrichment to cool the exhaust components. At low loads, EGR provides an additional benefit of reducing throttling losses.
In a modern EGR system, exhaust may be admitted into the intake air flow through an electronically actuated EGR valve. A motor-vehicle controller may actuate the valve in response to an output of an oxygen sensor coupled in the intake air flow. In some cases, the valve may be controlled in a closed-loop manner, such that the sensor output is regulated to a set-point value. Naturally, both the valve and the sensor are subject to degradation. In some cases, a degraded EGR system may be identified via diagnostic procedures enacted in the controller.
For example, U.S. Pat. No. 7,100,585 describes diagnosing degradation of an EGR valve based on a measurement of valve position. This approach requires a dedicated position sensor on the valve as well as a dedicated feedback channel in the controller. U.S. Patent Application Publication 2010/0294253 describes diagnosing degradation of the EGR valve by measuring a change in the intake air pressure on closing the valve. This approach may not be sensitive enough to detect a relatively small leak in the EGR valve.
Accordingly, the inventors herein have devised a sensitive and potentially cost-saving alternative method to diagnose degradation of an EGR valve using output from an oxygen sensor coupled in the intake air flow. One embodiment provides a diagnostic method for an engine having an EGR valve for controlling dilution of intake air. The method comprises, during engine fueling, indicating degradation of the valve when the valve is commanded closed but sensor output corresponds to an oxygen level below that of air. The method further comprises, absent engine fueling, indicating degradation of the sensor when the valve is commanded closed but the sensor output corresponds to an oxygen level below that of air. In this manner, sensor and EGR-valve degradation are unambiguously distinguished from one another, enabling appropriate corrective action to be taken.
The summary above is provided to introduce a selected part of this disclosure in simplified form, not to identify key or essential features. The claimed subject matter, defined by the claims, is limited neither to the content of this summary nor to implementations that address problems or disadvantages noted herein.